1. Field of the Invention
This invention relates generally to evacuated double wall tubing and methods of manufacture of such tubing.
2. Brief Description of the Prior Art
Double wall tubing or pipe is known for use in heat exchange operations and in insulating hot or cold liquids being conducted through such pipe.
There are a number of patents which disclose various types of double wall pipe or tubing and methods of manufacture.
Walsh U.S. Pat. No. 291,655 discloses double wall tubing or pipe having an annular space filled with a heat retentive material for heating rail cars at times when steam is not available.
Trucano U.S. Pat. No. 1,140,633 discloses double wall tubing or pipe which is evacuated to provide heat insulation and having a double wall coupling which is evacuated to prevent loss of heat at the coupling of the tubes together.
Adorjan U.S. Pat. No. 3,850,714 discloses coaxial tubes with the annular space filled with blown polyurethane foam.
These coaxial tubings or pipes are used for transporting hot liquids or gases.
McStravick U.S. Pat. Nos. 4,396,211, 4,444,420 and 4,480,371 disclose double wall tubing or pipe for use in thermally enhanced oil recovery in which the inner pipe is prestressed and the annulus filled with insulating material.
Stephenson U.S. Pat. Nos. 4,332,401 and 4,415,184, Stalder U.S. Pat. No. 4,340,245, and Hutchinson U.S. Pat. Nos. 4,477,106 and 4,453,570 discloses other types of double wall tubing or pipe where the inner tube is prestressed and the annulus supported by solid insulation.
Holbrook U.S. Pat. Nos. 4,470,188 and 4,538,337 disclose the use of techniques of heating or of drawing the inner tubing to prestress a double wall tubing or pipe.
Berti U.S. Pat. No. 4,560,188, Bonjour U.S. Pat. No. 4,609,373, and Neal U.S. Pat. No. 4,579,373 disclose various double wall tubing or pipe assemblies and special couplings therefor.
These patents do not deal with the problems of fabricating and evacuating double wall tubing where the inner tube or pipe is prestressed before the annulus is evacuated. Also, these patents do not address the problems of forming double wall tubing or pipe where the inner tubing or pipe is of titanium.
DeMaris U.S. Pat. No. 3,331,121 discloses explosion welding or bonding techniques for joining titanium to steel.
Doherty U.S. Pat. No. 3,645,435 discloses explosion welding or bonding techniques for forming end lap joints in tubing, no particular metal being specified.
The present invention is distinguished over the prior art in general, and these patents in particular by an evacuated double wall tubing made by a method which includes providing a first pipe of predetermined larger diameter and a second pipe having an O.D. substantially smaller than the I.D. of the first pipe. An evacuation opening is then formed in the first pipe. The second pipe is inserted inside the first pipe with an annular space therebetween. The pipes are welded together at one end. A stretching tool is secured to the other end of the second pipe after welding. The second pipe is then prestressed mechanically with the stretching tool an amount sufficient to prevent substantial buckling of the second pipe under normal operating conditions of the double wall pipe. The other ends of the first pipe and the prestressed second pipe are welded together, preferably by explosion welding, without the introduction of mechanical spacers between the pipes. The annulus between the pipes is evacuated through the evacuation opening, and the evacuation opening is finally sealed. The first pipe is preferably of steel and the second pipe is preferably of titanium. The pipes may be of a size and wall thickness sufficient for the double wall pipe to be structurally load bearing or may be of a size and wall thickness insufficient for the double wall pipe to be structurally load bearing, and the double wall pipe positioned with a sliding fit inside a third pipe of a load-bearing size.